An image sensor is a component of an imaging device generating an image in a cell phone camera or the like. Based on manufacturing processes and applications, the types of image sensors can be divided into a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal Oxide Semiconductor) image sensor. The CMOS image sensor is widely used as a general semiconductor chip manufacturing process due to its competitiveness in a high degree of integration, economy, and easiness in connecting with surrounding chips.
A conventional CMOS image sensor comprises a wiring part, a color filter part and a lens part stacked one after the other on a front surface of a silicon wafer. One problem of the image sensor having such structure is that reception of incident light by a light receiving element may decrease by metal wires inside the wiring part. Accordingly, what has appeared is a backside illuminated CMOS image sensor (BIS), in which a wiring part is disposed on a front surface of a substrate, and a color filter part and a lens part are disposed on a rear surface of the substrate.
FIG. 1 is a schematic plan view for describing a conventional backside illuminated CMOS image sensor. FIG. 2 is a cross-sectional view of the conventional image sensor, taken along A-A′ of FIG. 1. FIGS. 3 and 4 are schematic expanded views for describing issues presented by a boundary between a pixel region and a surrounding region of the conventional image sensor. Below, the issues that exist in the conventional image sensor are described in detail with reference to FIGS. 1 to 4.
As shown in FIG. 1, the conventional backside illuminated CMOS image sensor comprises a pixel region P and a surrounding region S. The pixel region P is where light entering from the outside is received, and the surrounding region S is a shielding region.
Referring to FIG. 2, the conventional backside illuminated CMOS image sensor comprises: a semiconductor substrate 710; a plurality of light receiving elements (PD) 720 provided inside the substrate 710 in the pixel region P; a wiring part 740 disposed on a front surface 730 of the substrate 710, an insulating layer 760 disposed on a rear surface 750 of the substrate 710, and a color filter part 770 and a lens part 780 provided on a rear surface of the insulating layer 760.
The insulating layer 760 includes a first shielding metal 760a formed on the pixel region P, a lower insulating layer 760b forming the same layer as the first shielding metal 760a, a second shielding metal 760c formed on the surrounding region, and an upper insulating layer 760d forming an uppermost part.
In the conventional backside illuminated CMOS image sensor, since light is illuminated from the rear side it is difficult to shield light entering a circuit in the surrounding region S. Thus, to maximize shielding, it is common to form the first shielding metal 760a and the second shielding metal 760c at different heights as shown in FIG. 2 such that the insulating layer 760 is relatively thick. Using this method also prevents decrease in shielding effects in the surrounding region S caused by a dishing phenomenon occurring in a direction of moving away from the pixel region P during planarization of the second shielding metal 760c. 
However, if the insulating layer 760 is constructed as above, the following problems occur. First, a height difference is created at a boundary between the first shielding metal 760a and the second shielding metal 760c. Then the upper insulating layer 760d becomes relatively thick as having to cover the thickness of the first and the second shielding metals 760a, 760b. In other words, the amount of light entering the light receiving element 720 disposed inside the substrate 710 in the pixel region P severely decreases.
Besides, referring to FIG. 3, if the thickness of the upper insulating layer 760d is made relatively small (or if the thickness of the upper insulating layer 760d becomes smaller than that of the first shielding metal 760a), a height difference is created at a boundary between the pixel region P and the surrounding S of the upper insulating layer 760d. Then the formation of the insulating layer 760 and the color filter part 770 becomes unstable, and the overall sensitivity unavoidably diminishes.
On the other hand, referring to FIG. 4, if the thickness of the upper insulating layer 760d is made thick, a refraction path of incident light inside the upper insulating layer 760d becomes relatively long. In that case, cross-talk between adjacent unit pixels occurs and the amount of light entering the light receiving element 720 inevitably decreases.
To address the abovementioned issues that the conventional backside illuminated CMOS image sensor has, the present invention is presented.